A Hercules is set to be deployed in South Sudan. Photograph: Corporal Amanda Campbell

Australia will provide two military aircraft to move United Nations forces into South Sudan and possibly help with evacuations, but is yet to consider providing any other assistance.

The acting prime minister, Warren Truss, made the announcement as he reaffirmed calls for Australians to try to access commercial flights to leave the country after fighting broke out in Africa’s newest country.

Truss said the Australian Defence Force (ADF) was preparing to provide a C-130 Hercules and a C-17A Globemaster to move UN personnel and equipment into South Sudan and undertake other tasks, potentially including evacations. They were operating in the Middle East and would be sent to the United Arab Emirates to await their future tasks.

“The precise deployment timeframe, which we expect will be just a couple of weeks, will be subject to the United Nations accepting the offer, and the aircraft will not be positioned to Africa until we are satisfied about the scope and nature of the mission and the risks have been properly assessed,” Truss said in Melbourne.

“We trust that this mission will be relatively brief; that we will be able to make a constructive contribution to this United Nations force and that peace will return to South Sudan just as soon as possible.”

Truss acknowledged the mission would carry risks, noting a US aircraft was fired upon as it tried to evacuate civilians on 21 December. Truss said the two aircraft would have appropriate crew to undertake the tasks, but no other additional Australian personnel were being deployed.

Asked whether Australia would consider sending additional resources or troops, Truss said: “We haven’t been requested by the United Nations to provide anything other than this airlift capacity. Australia has particular skills and is well equipped in that regard … but we’ve had no request for additional personnel and therefore we’re not considering any other commitments.”

Truss said about 70 Australians were registered with the Australian mission in South Sudan, many of whom were personnel from non-government organisations. The government was also aware of about 230 Australians who had already left South Sudan. He called on anyone remaining in South Sudan to make urgent arrangements to leave the country, saying some commercial flights were still available.

“We have reason to believe there’s perhaps 500 to 1,000 additional people, mainly dual nationals, who have not registered with the Australian embassies and therefore their presence and their activities in Sudan are not especially known. Now, those people too should take advantage of the commercial flights that are currently available so that they can leave the country and ensure their own safety,” he said.

Truss said fighting between government forces and rebel forces under the leadership of the former vice-president had caused “a degree of unrest in South Sudan over recent times”. He noted the referendum in January 2011 to create an independent South Sudan had attracted “overwhelming” support, but declining economic conditions had led to a deteriorating security situation.

“It’s of interest to note that there are some 70 countries currently involved in this United Nations mission. It is genuinely an international effort to try and support the United Nations’ youngest member in these times of difficulty,” he said.

A government statement said the ADF has a contingent in South Sudan of about 20 personnel, serving in key headquarters positions and aviation and logistics support roles and acting as military liaison officers. A small number of Australian police also remain in the country.